Geometry measurements
If you use bottom and pelagic trawl - or a twin trawl - the geometry measurements will:
- Measure if the trawl is skewed
- Allow you to correct the trawl geometry by adjusting the warp lengths
If you use a danish seine, the geometry measurements will:
- Detect variations in the distance from the centre of the headrope (or footrope) to either wing of the seine
- Detect differences in the rope lengths when the seine is closed (distances between the wings should then be close to zero)
Why is this important?
In order to maximize the performance and the catch efficiency, the trawl and the warps must be adjusted properly. This is essential to ensure a square trawl and minimum skew. This is also important for danish seines.
Any disturbance to the trawl movements may have a negative effect on the catch. This is particularly important on a bottom trawl. A skewed bottom trawl will result in unstable bottom contact, and create escape routes for fish below the gear and through the side panels. On pelagic trawls and danish seines a skewed net will make the gear unstable, and the catch be reduced.
Optimal trawl geometry is therefore vital for maintaining the catch efficiency.
What causes the gear to be skewed?
A trawl or danish seine may be skewed for several reasons. Some of these reasons are:
- Incorrect rigging of trawl, sweep lines or trawl doors
- Misadjusted warp lengths
- Side current
- Towing in steep or sloped edges
If the rigging of the trawl is otherwise correct, you can easily compensate for skew by adjusting either port or starboard wire length.
The basic facts
The Simrad geometry measurements are based on the fact that a correct rigged trawl and danish seine has exactly the same distance measured from the centre of the headrope or footrope to either door or trawl wing. These distances must be identical independent of weather conditions, sea state or water currents.
Example
On the left trawl, distance “A” on the port side is longer than distance “B” on the starboard side. The trawl is unbalanced.
On the right trawl, the distances "A" and "B" are identical, and the trawl is symmetric for maximum efficiency.
Measuring only the door spread (“C”) will not detect a distorted trawl geometry.
"Geometry" versus "Geometry Differential" measurements
Geometry measurements can be made in two ways.
- The geometry measurement is made up by measuring both Port and Starboard lengths simultaneously. Both lengths are sent to the vessel as individual measurements. The topside application (typically Simrad TV80) calculates the difference, and displays the geometry error.
- The differential geometry measurement is made the same way, but the sensor calculates the length difference. This value is sent to the vessel as a single measurement.
Using differential geometry allows you to save sensor battery, or you can add a second measurement to the sensor. However, differential geometry contains less information than the standard geometry measurement since the two lengths are not submitted.